How can rescuers ensure that they are providing effective breaths when using a bag-mask device?

The correct answer for above statement is:

sometimes it is identified as Ambu bag or generically as a standard resuscitator or "self-inflating kit", is a hand-held tool generally practiced to give positive pressure oxygenating to victims who are not exhaling or not breathing satisfactorily.So by using it one can observe chest lift with the breath.